As an apparatus for reducing the force necessary for a driver to operate a steering wheel when applying a steering angle to the steering wheels (normally the front wheels, except for special-purpose vehicles such as a fork lift), a power steering apparatus is widely used. For such a power steering apparatus, an electric power steering apparatus which uses an electric motor as the auxiliary power source is recently becoming popular. The electric power steering apparatus has the advantage that it can be smaller and lighter compared to a hydraulic power steering, control of the magnitude (torque) of the auxiliary power is easy, and there is minimum power loss for the engine. FIG. 46 is a schematic diagram showing heretofore known basic components of such an electric power steering apparatus.
Provided on an intermediate portion of a steering shaft 2 which rotates based on the operation of a steering wheel 1 is a torque sensor 3 which detects the direction and magnitude of a torque applied from the steering wheel 1 to the steering shaft 2, and a reduction gear 4. The input side of the reduction gear 4 is connected to the intermediate portion of the steering shaft 2, and the output side of the reduction gear 4 is connected to a rotation shaft of an electric motor 5. Furthermore, a detection signal from the torque sensor 3, together with a signal indicating vehicle speed, are input to a controller 6 for controlling the power to the electric motor 5. For the reduction gear 4, conventionally a worm reduction gear having a large lead angle and having reversibility in relation to the transmission direction of the drive force, is generally used. That is to say, a worm wheel serving as a rotation output receiving member is fixed to an intermediate portion of the steering shaft 2, and a worm of a worm shaft being the rotation force applying member, connected to the rotation shaft of the electric motor 5, is meshed with the worm wheel.
When in order to apply a steering angle to steering wheels 14, the steering wheel 1 is operated and the steering shaft 2 rotates, the torque sensor 3 detects the rotation direction and torque of the steering shaft 2 and outputs a signal indicating this detection value to the controller 6. In consequence, the controller 6 supplies power to the electric motor 5 so that the steering shaft 2 is rotated in the same direction as the rotation direction based on the steering wheel 1, via the reduction gear 4. As a result, the tip end portion (the bottom end portion in FIG. 46) of the steering shaft 2 is rotated with a torque which is larger than the torque based on the force applied from the steering wheel 1.
The rotation of the tip end portion of the steering shaft 2 is transmitted to an output shaft 10 of a steering gear 9 via universal joints 7 and an intermediate shaft 8. The input shaft 10 then rotates a pinion 11 constituting the steering gear 9, and moves a tie rod 13 back and forth via a rack 12, to thereby apply a desired steering to the steering wheels 14. As will be apparent from the above description, the torque transmitted from the tip end portion of the steering shaft 2 via the universal joint 7 to the intermediate shaft 8 is greater than the torque applied from the steering wheel 1 to the base end portion (the top end portion in FIG. 46) of the steering shaft 2, by the amount of the auxiliary power which is applied from the electric motor 5 via the reduction gear 4. Consequently, the force necessary for the driver to operate the steering wheel when applying a steering angle to the steering wheels 14 is reduced by this auxiliary power amount.
In the case of the electric power steering apparatus as described above which has generally been used up to now, as the reduction gear 4 provided between the electric motor 5 and the steering shaft 2, a worm reduction gear is used. However, in this worm reduction gear, there is unavoidable backlash. This backlash becomes larger with increase in dimensional errors and assembly errors of the worm shaft, the worm wheel, and the bearings etc. for supporting these members, being the components of the worm reduction gear. Furthermore, if a large backlash exists, the teeth surfaces of the worm wheel and the worm strongly collide with each other, with the likelihood of generation of a grating teeth hitting noise.
For example, if the vibrational load due to roughness of the road is transmitted from the vehicle wheel side to the steering shaft 2, then due to the presence of this backlash, a grating teeth hitting noise is generated. Moreover, due to the collision of the teeth surfaces of the worm wheel and the worm, the operating feeling when steering the steering wheel is impaired.
In order to address this, it has been considered to reduce the backlash by appropriate assembly, taking into consideration the dimensional accuracy of the components of the worm reduction gear. However, if backlash is reduced in this way, management of dimensional accuracy, and the assembly operation becomes troublesome, and leads to increased cost. Furthermore, recently there has been a trend to increase the auxiliary power. Therefore the friction between the teeth surfaces of the worm wheel and the worm is increased so that backlash is more likely to occur. If the teeth hitting noise based on this backlash leaks into the cabin space of the vehicle, it is annoying to the occupants.
The following Patent Documents 1 to 4 are prior art documents related to the present invention.
[Patent Document 1] Japanese Unexamined Patent Publication No. 2000-43739.
[Patent Document: 2] Japanese Unexamined Patent Publication No. 2002-37094
[Patent Document 3] Japanese Unexamined Patent Publication No. 2001-322554
[Patent Document 4] Japanese Unexamined Patent Publication No. 2002-67992